« PreviousContinue »
69. Batching, Mixing, and Placing Concrete. - Batching of concrete aggregates was accomplished by means of a batch plant equipped with three storage bins, a sliding weighing hopper, and 3-beam scales. Aggregates were conveyed from stockpiles to the storage bins by the use of a dump truck and a crane equipped with a clamshell bucket. Three-sack batches of aggregate were weighed and transported to the mixer in dump trucks equipped with compartments. Since the batch plant was located only a few hundred feet above the dam, the greatest length of haul was not over one-half mile. Routine gradation and moisture tests were performed on samples of aggregate obtained at the batching plant. The results of these tests were summarized for transmittal with the monthly concrete control reports.
Concrete mixing was accomplished by means of a 16-cubic-foot mixer located at the structure site (fig. 27). Cement was added to batches at the mixer. Water for mixing concrete was pumped from Conejos River. A pumpcrete machine or a crane equipped with a concrete bucket was used to transport concrete from the mixer to the structure being placed. Methods and equipment used in placing concrete in the outlet tunnel are described in section 72. Consolidation of concrete in the forms was accomplished by means of immersion-type electric or air-powered vibrators. In addition to the above, an electric form vibrator was used in the tunnel.
When required, aggregates were heated in the batch plant bins by a steam boiler and pipes. Water was heated at the mixer in order to maintain the concrete at specified temperatures. Protection from freezing weather of newly placed concrete was accomplished by covering structures with tarpaulins and heating with oil-burning salamanders, butane heaters, floodlights, or oil-burning lanterns.
Curing of all concrete except that in the valve house structure was accomplished by application of curing compound. Concrete in the valve house structure was water cured by using soil-soaker hose or burlap and kept continuously moist for a period of 14 days.
70. Air Entrainment. - An effort was made to maintain the amount of air entrained in the concrete at 4 percent, plus or minus 1 percent, after final placement in the forms. Air-entraining agent was pumped into the mixer simultaneously with the mixing water. The amount of air entrained was checked frequently at the mixer by laboratory personnel. In determining the air content of the freshly mixed concrete, the unit weight method or the pressure meter test method was used. The results of air-entrainment tests were summarized and included in the monthly concrete control reports.
71. Concrete Mixes. - Routine 6- by 12-inch concrete test cylinders were cast in cans during concrete placing operations. These cylinders were sent to the central Bureau laboratories for 28- and 90-day compressive strength tests. A summary of concrete mixes used and results of strength tests are included in the following table:
F. Concrete Structures
72. Tunnel Lining. - Concrete operations for the tunnel lining were started with placing of the invert in the high-pressure section beginning at station 1+80 on September 6, 1949 (fig. 30). During this placement, screeding was attempted with a "boat" made up of steel plate and pulled with an air-powered winch. This proved unsuccessful; the winch had a tendency to jerk, causing the boat to ride high which, in turn, resulted in the need for considerable hand labor in striking off to the required grade. Consequently plywood forms were fabricated in 6-foot sections and the remainder of the invert placements were formed. As soon as practicable after each placement the forms were removed and finishers repaired any surface imperfections and gave the surface the proper finish. It was necessary to prevent water dripping from the roof of the tunnel and pitting the finished surface. This was accomplished by suspending a tarpaulin over the area until the concrete had attained its final set. There was some difficulty at first in obtaining a seal under the forms, but by experimenting with placing procedures this was elimated. Thereafter, invert placements continued without incident.
Equipment utilized in performing this work consisted of a 16-cubic-foot concrete mixer which was charged from batch trucks that had been filled at a central batching plant (fig. 27). Concrete was discharged directly into the hopper of a 6-inch pumpcrete machine from a 6-inch discharge line. This machine proved satisfactory until invert placements were completed in the high-pressure section and arch placements were begun. The first placement attempted in the arch section was made on September 30, 1949, at station 7+24. After attaching the "slick line" in position at the top of the arch and after a very small amount of concrete had been pumped, the pumpcrete machine labored excessively and finally stopped with a plugged line. The placement was abandoned and the contractor ordered an 8-inch pumpcrete machine and a 7-inch line, but these resulted in no improvement in operation. Mixes were changed and slumps varied, within reasonable limits, with no appreciable improvement.
Figure 27. --Equipment used at outlet portal of tunnel for concrete operations. A 16-cubicfoot mixer was used for mixing the concrete, and the concrete was unloaded into a pumperete machine and pumped through a 7-inch line to forms within the tunnel. 5-SL-510, November 28, 1949
Figure 28. --Pumping concrete into grout cap trench of the right abutment by means of a pumpcrete machine. The truck-mounted crane was used to transport concrete from a 16-cubic-foot mixer to the hopper of the pumpcrete machine. The small end-dump trucks hauled weighed aggregate from nearby stockpiles. 5-SL-885, October 26, 1950
It then became necessary to install an air line to the pumpcrete discharge just back of the slick line as a booster. This arrangement, although not ideal, proved to be the only workable solution and was utilized as required.
Prefabricated steel forms, made up in 7- and 5-foot sections, were used in the highpressure section of the tunnel. Throughout the curve it was necessary to space sections and use plywood filler between the form joints. Considerable difficulty was experienced at first because of the concrete adhering to the steel form skin plate. A number of recommended and patented types of form oils were tried until the situation was corrected. The form oil which finally produced the desired results was composed of four parts of a straight petroleum oil and one part of paraffin. Placements in the arch section of the high-pressure tunnel were completed November 1, 1949, somewhat behind schedule, and all placing equipment was moved to the downstream portal to begin operations in the horseshoe section.
Since most of the placing difficulties were solved in the high-pressure section, placements were carried out in the horseshoe section (fig. 29) with only minor delays. Invert placements were completed November 9, and the arch section November 29, 1949. A 30foot section of prefabricated steel form was used in this portion of the tunnel arch.
Freezing weather in October required heating of aggregates and mixing water. Oil burners were placed at intervals throughout the tunnel in November, to maintain an average temperature of approximately 60° F.
Upon completion of lining operations, forming was started in the gate chamber. The substructure was placed and the superstructure completed 9 days prior to the enforced winter shutdown on December 24, 1949. Because of the delays mentioned in preceding paragraphs, overall placing operations fell considerably behind the contractor's planned operation schedule, which had called for completion of all concrete placement in the outlet works during the 1949 construction season.
73. Grout Cap. - Concrete placement in the grout cap was initiated August 12, 1949. Placements were made intermittently throughout the 1949 construction season as equipment and men became available. The objective was to complete concrete placement in the lower portions of the dam and dike so as to prevent any delay in fill placement which was expected to begin early in the 1950 construction season. With the completion of the 1949 season, approximately 35 percent of the grout cap concrete was in place. Placements were resumed June 15, 1950, and were completed October 25, 1950.
In accessible areas, placing was carried out with the use of a 10-cubic-foot concrete mixer and a small crane. In inaccessible areas or where placements were made high on abutment slopes, it was necessary to place with the pumpcrete equipment used in lining the tunnel (fig. 27). This arrangement permitted the grouting to be completed well in advance of fill operations and insured against any delay in fill placements.
No unusual problems arose during the period required to complete this work, with the exception of protection against freezing weather, which was necessary from the first of October during 1949, and from September 11 throughout the 1950 season.
74. Trashrack and Conduit Section. - The trashrack proper and the transition to the conduit section were placed September 27 to October 24, 1949, during slack periods in tunnel lining operations. Placement in this portion of the structure was performed with tunnel equipment (that is, a pumpcrete machine and a 16-cubic-foot concrete mixer).
The conduit section, extending from the trashrack transition to the tunnel portal, was placed in November 1949 with a 10-cubic-foot concrete mixer and a truck-mounted crane. The only problem here was providing adequate protection from freezing, which was accomplished by covering with tarpaulins and heating with oil-burning salamanders.
75. Spillway Bridge and Crestwall. - Concrete operations in the spillway section were started October 10, 1950, with placing of the bridge piers (figs. 31 and 32). The bridge abutments and parapet walls were placed next, and work was completed with the placing of the bridge deck and curb walls November 11, 1950. The spillway crest wall,